docs: def build_pair(self, losses, stats):
          """
          :param losses: a list of lists with R elements
          :returns: two arrays of shape (P, R) and (P, S) respectively
          """
          P, R = len(self.return_periods), len(self.weights)
          assert len(losses) == R, len(losses)
          array = numpy.zeros((P, R), F32)
          for r, ls in enumerate(losses):
              ne = self.num_events.get(r, 0)
              if ne:
                  array[:, r] = losses_by_period(
                      ls, self.return_periods, ne, self.eff_time)
          return self.pair(array, stats)